The origin of extrachromosomal circular copia elements.

Cloned extrachromosomal circular copia elements were studied by nucleotide sequence and restriction enzyme analysis to determine their mechanisms of formation and their possible roles in copia transposition. Rearranged circular copias containing inverted segments flanked by 5 bp sequence duplication were observed, suggesting that circular copias are capable of integrating into their own sequences. Such copia circles are analogous to similarly rearranged retrovirus circles, strengthening the relationship between the copia-like elements and retroviruses. There is marked sequence heterogeneity at the junction between the fused terminal direct repeats of seven copia circles. The junctions contain 0-15 bp inserts, the sequences of which are inconsistent with the creation of these particular molecules by reverse transcription.